The ESRI says the calculations for free water for children may not be correct.
Last week the Commission for Energy Regulation announced the allocation was to be 21 thousand litres per year for each child.
The Department of the Environment announced in May that the free water allocation for children was 38 thousand litres.
But last week, when charges were formally announced, the child allocation had been cut to 21 thousand litres.
An associate research professor for the ESRI has told the Sunday Times the calculation on how much water children use may be wrong.
Edgar Morgenroth said he can't stand over Irish Water's data as the sample of metered households used for the research may be unrepresentative.
He says analysis depends on the type of estates where the meters are installed, the time of day the survey was carried out and may not have factored in seasonal variations.
He has recommended that Irish Water and the CER "urgently revisit" assumptions on water consumption.
